Detection of toxic chemical substances in the atmosphere is an important guarantee of our safety both in civilian or professional life. Limiting average exposure values are moreover already established for a great number of chemical agents. In this background, development of performing chemical sensors, i.e. which transform chemical information into a useful analytic signal, is therefore an absolute necessity.
However, strong constraints restrict the manufacturing of such chemical sensors, the latter having to be both sensitive and selective.
Indeed, the sensor should be very sensitive to the target chemical agent, i.e. be capable of delivering a response for contents below the limiting contents (ppm or ppb depending on the gases).
The sensor should also be selective, i.e. it should be able to distinguish between several chemical agents.
To these fundamental constraints, other more secondary issues may be added to the specifications sheet.
Thus, it is preferable that                the sensor have low bulkiness, in other words it may be miniaturizable, in order to carry out analysis directly in the medium to be analyzed instead of proceeding with sampling;        the sensor should be simple to manufacture in order to have a production cost as low as possible;        the sensor during operation should consume little energy and this for increased portability;        that the sensor may be used several times (reversibility principle of the sensor).        
Many toxic gases are targeted by devices of the sensor type: volatile organic compounds, ammonia, nitrogen oxide, chlorinated compounds, dimethyl methylphosphonate (DMMP), hydrogen, methane . . . . The state of the art on sensors of chemical agents presented hereafter more particularly relates to the devices allowing detection of chlorine gas and sensors based on carbon nanotubes.
The chlorine concentration value which may cause an immediate hazard for life and health is about 10 ppm. The limiting weighted average exposure of chlorine (8 hrs of exposure per working day) has been evaluated as 500 ppb for certain public bodies such as the Commission de la Sante et de la Sécurité du Travail (Occupational Health and Safety Commission) (Canadian Organization). This is why it is required to ensure detection of this gas at concentrations of the order of hundreds of ppb, or even below.
Generally, sensors of chemical agents consist of two main components: a recognition system and a signal transducer. They are classified according to their transduction method: electric, calorimetric, electrochemical, optical, mass or magnetic transduction and affect four different types of markets which are the health, environment, industry and defense markets.
Different chlorine-sensitive sensors already exist: mention may be made of potentiometric sensors, ionization sensors, optochemical sensors and resistive sensors from semiconducting materials.
Potentiometric sensors are based on making a solid electrolyte, for example SrCl2—KCl or PbCl2—KCl or BaCl2KCl. The operating principle of these sensors is based on the measurement of a potential, which varies depending on the chlorine content. Typically, the responses of these sensors are studied for ranges of concentrations above 1 ppm. The main drawback of this type of sensor is therefore their low sensitivity. Moreover even if detection is reported to be at room temperature in certain studies [1], the system is generally heated to a temperature typically above 300° C.
The principle of ionization sensors consists of ionizing chlorine when the latter passes close to a source of electrons (thermo-ionic emission). The chlorine content may be estimated from the collected current. With this technique, it is possible to detect very low chlorine contents, below 100 ppb, independently of temperature and humidity. The drawback of this method however is the non-portability of the measurement method, preventing a measurement in situ.
The principle of optochemical sensors is based on the study of optical properties of thin films sensitive to the gas to be detected, these films may be sensitive to a single gas in particular (principle of the selective sensor). The adsorption or the reaction of the gas at the sensitive film leads to an effective change in its fluorescence or absorbance properties, at specific wavelengths. With the rate of change of the absorbance or of the fluorescence, it is possible to trace back the concentration of the studied chemical agent. In the specific case of chlorine, the shaped sensitive films may be phenylporphyrin, o-toluidine, tris-bipyridyl ruthenium complexes or else nanoporous films containing specific molecules or ions (Br−, I−) [2]. With these opto-chemical devices, it is possible to detect chlorine at a low content, down to 50 ppb [2]. Moreover, let us note that the detection occurs at room temperature. The drawback of this type of sensor is the manufacturing cost of a complete device which notably comprises the sensitive film, a diode and a detector.
For resistive sensors from semiconductors, it is the measurement of the resistance of the device which allows detection of the gaseous agents. This is the type of chemical sensor which is most frequently encountered in the literature, notably for detecting chlorine. The adsorption of a gas at the surface of the semiconductor may induce a charge transfer which leads to the generation of carriers and to a modification in the conductivity of the system. The sensitive layers may be organic semi-conductors such as metal phthalocyanin or else metal oxide semiconductors such as InO2, WO2 or SnO2. Detectors based on semiconductors are devices which are very sensitive to chlorine. The literature reports devices having very low detection thresholds: from 50 ppb [3]. However, let us note that the detection only becomes effective when these sensors are heated, typically at temperatures above 150° C. The fact that they do not operate at room temperature is their main flaw. Moreover, it is noted that in the specific case of chlorine the selectivity of this type of sensor is never mentioned.
To summarize, among the different devices described above, none of them totally meets the criteria established earlier.
Since a few years, a novel very promising material, the carbon nanotube, has been used in many studies, as the sensitive component in chemical sensors.
The devices based on carbon nanotubes benefit from the singular properties of this material. Firstly, the carbon nanotubes have a very large specific surface area, which may attain 1,500 m2/g and which thereby provides a very large surface for interaction with the surrounding atmosphere. Secondly, carbon nanotubes have a very high electric conductivity which proves to be extremely sensitive to the adsorption of molecules on their surface, and therefore more generally to their environment. The combination of these properties makes a carbon nanotube a very interesting material as a sensitive component in chemical sensors. Further, the small size of the carbon nanotubes is favorable for making miniaturized devices.
Many types of chemical sensors based on nanotubes are listed in the literature. These may be sensors based on the CNTFET (Carbon NanoTube Field Effect Transistor) principle, sensors of the resistive type based on the measurement of the resistance of the system, acoustic sensors, sensors for measuring impedance, ionization sensors . . . . Only the sensors of the CNTFET type and of the resistive type based on carbon nanotubes will be explained hereafter, both types being the closest to the present invention.
A first approach consists of dispersing (single-walled or multi-walled) carbon nanotubes in a sensitive array. The operating principle is the same as that of the resistive semiconductor sensors (measurement of the change in resistance of the system). The studied matrices are either tin oxide (SnO2), or polymers (PMMA, polystyrene) or further tungsten oxide (WO3) [4,5]. The gases detected by these devices are nitrogen dioxide (NO2), tetrahydrofurane (THF), chloromethanes and other volatile organic compounds (VOCs). By comparing the devices with a control (without any nanotube), it appears that sensors including nanotubes are much more competitive, notably in terms of sensitivity [4]. The detection thresholds observed at room temperature may at best be less than 1 ppm (500 ppb for NO2 in reference [5]).
Another approach is interested in direct synthesis of carbon nanotubes within the «sensor» device. The principle of this type of sensor is shown in patent application US 2007/0145356 [6]; however no detection example by means of such a device is shown in this document. In this type of device, the growth may be controlled so as to obtain a unique single-sheet carbon nanotube providing the junction between two metal electrodes, deposited by lithography. The silicon substrate, which may be biased, acts as a grid. First studies, carried out in 2000 by Kong et al. on these devices measure the conductance of the nanotube at room temperature versus the gas environment [7]. The observed detection limits are 2 ppm for NO2, and 0.1% for NH3. By using the same type of device, but this time with carbon nanotubes covered with a thin layer of polyethyleneimine, Qi et al. were able to reduce the NO2 sensitivity threshold to 100 ppt [8].
Multi-walled nanotubes may also be used as a sensitive component directly synthesized within the device. In the same way, as in the previous paragraph, multi-walled nanotubes may be synthesized laterally and with a low density between two electrodes [9]. These sensors, notably sensitive to ammonia, have a detection threshold of 50 ppm [9]. In most of the devices, the elaboration of which comprises a phase for synthesis of multi-walled nanotubes, the growth of the nanotubes is massive and is directly carried out on a substrate (Si, SiO2 or Si3N4) from a thin catalyst layer (a few nanometers). Measurement of the resistance of the system is carried out by means of interdigitated electrodes deposited beforehand (under the catalyst layer) or else by depositing two metal electrodes over the film formed by the nanotubes. This type of sensors may be heated in order to improve the detection performances. In the example shown in reference [10], the device operates at a temperature of 165° C. and exhibits a very low detection threshold of about 10 ppb for nitrogen dioxide. Other gases studied by means of the same sensor, ethanol and benzene, as for them, have much higher detection thresholds (>>1 ppm). The detection thresholds for methane (CH4) and for hydrogen (H2) measured by other groups with the same type of device are also very high (>1,000 ppm). Among the devices shown in this paragraph, only the sensor sensitive to methane is investigated at room temperature [11].
A third approach with several steps consists of carrying out in a first phase, the synthesis of the carbon nanotubes, and then of dispersing them in a solvent, and finally depositing the nanotubes from the dispersion obtained beforehand on a network of lithographed electrodes, the evolution of the resistance between the electrodes then allowing detection of the pollutants targeted by the study. A device consisting of single-walled nanotube aggregates achieving the connection between interdigitated electrodes was also developed. With this device which operates at room temperature and in which the nanotubes are covered with a layer of chlorosulfonated polyethylene polymer, the observed chlorine detection threshold remains rather high: >2 ppm [12].
The multi-walled nanotubes may also be dispersed and then deposited randomly on electrodes [13]. With such a device (with two electrodes), Wang et al. report detection of ammonia at room temperature from 5 ppm onwards.
Let us note that in this third approach, the carbon nanotubes deposited on the electrodes during the shaping of the sensor are oriented randomly. A few investigations use the dielectrophoresis technique for aligning the nanotubes in a preferential way perpendicularly to the electrodes. The dielectrophoresis technique consists of applying an alternating electric field between the electrodes. By using this deposition method for multi-walled carbon nanotubes, Suehiro et al. made a device with which ammonia was able to be detected from 500 ppb onwards at room temperature [14].
A few patent documents have chemical sensors based on the measurement of the conduction of single-sheet carbon nanotubes, the nanotubes being individual or organized as networks between lithographed electrodes [15-17]. In these documents, a sensitive layer is added to the nanotube(s) connected to the electrodes. Patent application US 2006/0263255 reports the use of single-walled nanotubes covered with palladium particles for detecting hydrogen [17]. International applications WO 2005/026694 and WO 2005/062031 show CO2-sensitive devices based on single-walled carbon nanotubes covered with a layer of polyethyleneimine [15,16]. The polymer is deposited by simply adding a drop of solution onto the device. Other functionalizing agents are shown in these patents (metal carbonates, aromatic compounds, various amine compounds . . . ) but no example is associated with them. It is important to note that the functionalizing agents are deposited by impregnation (physical absorption) and are not grafted to the nanotube in a covalent way. U.S. Pat. No. 7,312,095 proposes functionalization of single-sheet nanotube(s) integrated into a sensor device by causing a strong current to flow in the latter while exposing it to a reagent assumed to be sensitive to a specific gas [18]. However no example is shown in the latter patent.
The design of a device meeting the whole of the constraints listed earlier still remains presently a challenge which is only partly faced. The manufacturing of such a sensor therefore is a very important issue.